My connection is specified to 10 Mbps. But it depends on how
large the files are. Most of the files from DConf are under
around 350MB in HD quality. On the other hand, Andrei's talk
from LangNext 2014 is 1.3 GB and 48 minutes long while the
talk by Bjarne is 2.8 GB and 68 minutes long.

There are also 740 and 65.8 MB encodings of Andrei's talk that
are perfectly usable. I should know, as I downloaded the
latter.

Same for Bjarne's talk, which I haven't downloaded.

Sorry, I just noticed that you were only talking about "HD
quality." I don't know where you're getting the 350 MB figure,
as all the HD recordings on archive.org are about 6-800 GB, but
yeah, file sizes will vary based on the type of HD resolution and
encoding used. I wouldn't call any hour-long video encoded into
350 MB "HD quality" though, as it's likely so compressed as to
look muddy.